Direct reading capaciometer



April 25, 1933- P. s. EDWARDS ET AL 1,905,349

DIRECT READING CAPACIOMETER Filed June 7, 1929 llllllllll Paten-ted Apr. 25, 1933 ram. s. nnwnms m coNsrmiN n. minstrmsco. or nano-N, -omof f y ninncr`nmn1ne caracrilmm r '1"App1ioatlon'1ed`l'une 7, 1929. Serial 369,282.1v f

l (namur man' ma Acro! men a, ilsaaas mxnsnunn. so. laas;4 arbic-3,1m), Y

The invention described herein, ifpatented,

l may be manufactured and used byor for the Government for governmental purposes,

without the paymentto-us of any royalty .5 thereon.

trical measuring instruments and morepariticularly to a method and apparatus ffor measuring capacitances `by. direct reading ofv 10. an indicating instrument. r l

.Heretofore the measurement of capacitors -has involved as: a` general rule complicatedv bridgeA arrangementsA and necessitated care- .ful adjustment and` rather extensive .compu- 15 tation to attain the inalresult. v

The major object ofthe present invention is to avoid all suchcomplicatedmethods of measurement by providing an apparatus whereby the capacity offa 'ven element 2 maybe immediately read `orindicated 1n a single instrument. .l 5 s Another Objectis to. provide anapparatus in which the ca acity of a given capacitor maybe indicate directly in microi'arads.` 25 A'further object is to provide a device ci,

the character described in which the ca acity of a given element may be quickly indlcated without` resorting. to mathematical calculations. e' f In order to more clearlyv explain the invention, preferred `mechanical embodiments of it are yshown in the accompanying drawing, in which: Fig. 1 is a diagrammaticillustration of 35 the general circuit employed.

Fig. 2 is a' schematic view of' theoperation of certain portions of the circuit. Fig. 3 is an illustration ofthe plate current characteristic of the tube.

*4 c Fig. 4 is a modification yof the form' of thek invention shown Vin Fig. .1. The present invention is acontinuation '1n art of applicationASerial No. 305,753, filed 45 eptember 13, 1928.

As indicated above, the determinationof` the capacity of a, given element hasinvolved considerable mathematical calculations. 'The results obtained are not always precisely ac-fr curate because of the necessityiof vassuming V .y .1 the constancy of-certain factors,lfsuchasvthe 5o' i This invention relates Ain lgeneral to elecfv thicknes'sof Vthe dielectric andthe"uniformity` of the dielectric constant. `I`urthern1ore the calculations-.may become rather -extensive as inntheycase of the deferlinatiOn1 ofl the `ca'`- pacityof variable 'condensers which "comprise 86" semi-circular plates, or 'more particularlythe:Y determination ofthe capacity of the so c'alled'` V v straightlinefcondensers. l

- In accordance with 'the present 'mventionji l, v

. j -As disclosed' inFig. 1, the'instrument em-v bodying the principles ofthe present inven y tion comprise a vacuum tube including the l filament' 1, grid 2 and plate 3. 'As shown, 7 these are enclosed in an evacuated'envelope Aor container '4 in 'the 'usualmannerf C0n` -.nected to the tube 1s a .low losscircuit comf prising a large capacity wfand'small 'indue- 1 v tance -6. This circuit, knownla'slalow loss 75 tank circu'it,flargely` controlsffthe frequenc at which the tubeoscillates, as. explained fu Vly in the prior application.

- Con ductively Vassociated with "the filamentl 1 are thev high frequency choke coils 7 and 8.10 These are composed of a' relatively f large number of turnsof wire and areincluded re# spectively in the grid and plateicircuits of the tube. Inoperation these chokecoils in? duce-two driving 'electromotive forces whichl 85 are substantially 180 out lvof phase; The .choke coil 9, similar in construction'to the coils* 7 and 8,' serves to maintain the filament: 1' at a high` potentialA with 4respect to thev ground.` In operation, the geometric sum of N the condenser-is of a large value, the inter-- electrode capacity of the tube becomes relatively substantially negligible in effect and hence replacements of the tube or minor variations .m the characteristic of a given tube during normal `use willv not appreciably ef-` fect the frequency of the oscillations generated. rvAs a result the tube is peculiarly susceptible to extraneous or interposed electri cal effects.` Y, f

.The grid circuit is rovided with a block- 25 ing condenseril' an a leak resistance 15. The `value of the condenser 14 and-.resistance 15 isfsovchosen that the high frequency oscillations will periodically charge thef'condenser to a certain value and thendisch'arge 5 through the leakresistanee Ato the ground. Therefore the condenser 14 and` resistance 15, together with any other additional coupled capacitors or resistances, produce a periodic interruption of they hi h frequencyroscilla- 35. tions. y'Iv-.heperio'd of the parasitic discharge is controlled bythe time constant CR which may'vary, depending onthe values chosen,- from a few cycles per second to several thousand or more.

4 The effect of the associated condenser and leak resistance is indicated schematically in Fig. 2, in which the shape of the modulated high'frequency oscillation isl shown. The periodic charge anddischarge of the block- ,ing condenser is indicated by the envelope ofl the amplitudes of the highfrequency oscillations. As indicated above, the frequency -of thisperiodic' charge and discharge depends uponv the chosen values of the condenserand leak resistance,assuming the other values of the .circuit to be constant; hence for a given large value of the grid leak the frequency is low and the grid of the tube is strongly biased. The resultant plate current consequently is very small and will remainat l' this low value until some external orinternal cause is made to eect the circuit. v

Conversely, a decrease in the value of the grid resistance will tend to increase the fre# cpliency of the periodic discharge and since t e amplitude of the high frequency oscillations cannotreach high values, the grid bias decreases lwith the resulting increase in plate current. Similarly a decrease in the value of the blocking condenser will have the same effect. In other words, the value of the blocking condenser may be varied to chop oil' high frequency oscillations and produce a strong negative bias on the grid of the tube.

This effect is shown schematically in Fig. 3 in which the operatingpoint is indicated as being automatically shifted on the static characteristic of the tube toward the negative values of the grid voltage and the consequent plate current. As shown, when the operating point is at a the corresponding plate cur- V rent A is quite small in value as 'compared with the maximum or saturation current of thetube.' j 1 It will thus be seen that a variation in the value of the grid condenser is always accompanied by Ia corresponding variation in the plate current. This effect is due initially to thevariation of the frequency of the periodic discharge, for as the frequency of this discharge' increases, theduration Vof a complete charge and discharge is.- small and the amplitude'of the highfrequency oscillations does not reach a highvalue and the bias effect is therefore reduced. Concomitantly the plate current takes on anew value determined by the'rcorrespondingffnew value of the grid bias.

It will be perceived that if, in vplace of a given grid condenser v14, a set of condensers' of well known value be inserted successively,

becomes a"means'.of,.ldirectly indicating'the capacitance-of any .given element in termsof its ultimate `value'and this applies equally whether the elements to'be tested com rise a simple arallel `plate condenseriso atedl thin circu ar disc, concentric s heres, or coaxial cylinders. In short, the mstrument is capable offmeasuring precisely the actual capacity of any element which 'is desired to be tested. Furthermore, as will be appreciated, the instrument presents an excellent means of calibrating any type of variable condenser. y i

It will be appreciated that although the indicating meter 16 is shown as directly connected in the plate circuit, it may be utilized elsewhere; for example if desired the output circuit may be connected to amplifying stages Vand the indicatingmetery connectedin the output circuit of the final amplifying stage.

It will be appreciated also that other specific methods of utilizing the rinciples involved in this invention may eiemployed.

Ya ed in the plate circuit of the tube.

An exam le of such is shown in Fig. 4. In this case t e grid leak is connected to a switch 18 which is adapted to contact either with a variable condenser 17 or with the condenser 5 of ,unknown value 14. The other portions of the" circuit are similar to those described in Fig. 1. In the second method, employing the device shown in Fig. 4, the capacitor which is to be tested is inserted in the circuit by suitably connecting to the terminals M and N. The designation 14, it will be understood, indicates not only a single condenser but any arrangement of plural condensers. After the unknown condenser is interposed in the circuit as described, the input circuit through this element is closed by throwing the switch 18 to the terminal connectedto the unknown condenser.

The interposition of this condenser in the 20 circuit will, as described above,eiect the parasitic oscillations in the input circuit and cause a corresponding variation in plate current. The reading on the meter 16 is therefore a direct function ofthe capacitance of the element 14. This reading is noted and the switch 18 then thrown to the terminal connected to the calibrated variable condeiis` er 17. This variable condenser isr then adjusted until the pointer on the meter 16 indicates the same value as was obtained upon the testing of the condenser 14. y

Since the condenser 16 is calibrated in terms of capacity, the value of the unknown capacity may therefore read directlyfrom this condenser. This method presents the advantage of permittin the utilization of an ordinary ammeter for t e current indicating instrument 16 and also permits the use of the ap aratus for other uses.

t will be appreciated that we have provideda capaciometer in which the value of an unknown capacitance may be quickly and accurately determined and which determination is not effected by any minor variations 45 in the characteristics of the tube. Due to the very great sensitivity of the circuit, very small changes in capacitance may lbe measured and', as explained, the resultant plate current may be amplified to any desired degrec. The device may be made up to include a specially calibrated dial as in Fig. 1, or may be employed with standard instruments available on the market.

While preferred embodiments have been shownand described, it is to be understood, that these are given by way of explanation and are to be considered as illustrative and not restrictive.

We claim:

1. A capaciometer comprising a vacuum tube oscillator, means to connect a capacitor to be tested in the grid circuit of the oscil-` lator, and means to directly indicate the value of the capacitor com rising a meter connect- 2. A capaciometer.comprising. a vacuumV tube oscillator having an vassociated low loss tank circuit, means to l,connect a capacitor tobe testedin thegrid circuitbf the tube to' thereby vary-the value `of\t\he plate current and means to indicate suchjvalue ofthe plate current. I Y 4 y 3. A capaciometer comprisingl a self-modulated oscillator including alvacuum tube and anassociated low lossv tankV circuit, a capacitor connected to Athe grid of the tube, a'leak vresistance associated withthe capacitor and a current meter` connected'inathe"plate circuit directlyresponsive to variations vin the value of the capacitor.

4, A capaciometer comprising a vacuum tube and a low loss tankcircuit coupled to the input circuit of the tube, means to modulate the high frequency oscillations at audible frequency including a condenser and leak resistance connected tothe grid, a current meter connected in the-plate circuit, calibrated in capacity units and adapted to directly indiy cate the. capacitance of the condenser.

5. A capaciometer comprising a vacuum tube,atank circuit including a large capacitor and small inductance connected to the input of the tube, means to connect a condenser to be tested in the grid circuit, a leak resistance associated with the condenser, a current meter calibrated in termsof capacitance connected in the plate circuit and adapted to indicate the value of the said condenser.

' 6. .A capaciometer comprising a self-modulated vacuum tube oscillator, a current meter associated with the output of the tube, a resistance leak connected to the grid, a calibrated variablecondenser and a xedcondenser arranged for successive connection to the grid and a switch adapted to connect one of the condensers to the grid.

7. A capaciometer comprising a vacuum tube having a filament, grid and plate, a tank circuit comprising a low resistance, large capacitance and low losses connected across the grid and plate, a coil between the filament and grid and a second coil between the filament and plate of the tube, means to connect a capacitor to `be tested in the grid cir- *means to indicate the value of the capacitor comprising a meter connected in the plate circuit of the tube.

t -9. A capa'ciometcr comprising afvacuum tube oscillatorhaving an associated 1ow-1ossf tank circuit, a resistance leakconnected to the grid, a calibrated yvariable condenser and 5 a' xed condenser arranged for vsuccessiveconnection to theA grid, a switch adapted to successively connect the condensers to. the grid and a. meter associated with the output of the tube forindicating the capacity of the. m unknown condenser. f,

In testimonywhereofwe aiix our signa-A tures.- l r PAUL S.' EDWARDS.:

CONSTANTIN-DBARBULESCO. f 

